Can you run a program to check for values in other existing lists prior to sending emails. For examples, I have leads coming in via one program but I wanted to check if a specific lead value exits in a different list before executing an email.

Yes you can do this and there are several ways, depending on the exact nature of your flows:

If your campaign is based on Campaign IS REquested, then just add filters below the trigger to block the group.

Flow: use Choice steps to check for a Value or Membership before sending. This is less desirable because it should have blocked those people at the Smart List. Best used when you have several options to send.

Check in Requesting Campaign - use a Choice Step

Use a traffic cop (don't hate me!) - this checks for data value changes and routes the lead appropriately.